refactor: Prefer use of RequestError and next() function on middlewares

Added:
- New error types such as `IntegrationNotFoundError`,
  `WorkspaceNotFoundError`, `AccountNotFoundError' and more.

Refactored:
- Refactored most of the middlewares and very little number of helper
  functions to use RequestError
- Deleted unused imports

Changed:
- Some of the error types in middlewares changed to more related error
  types.
- Environment variable of 'VERBOSE_ERROR_OUTPUT' changed to more
  reliable validation method in `config/index.ts` as per @dangtony98
  requested.

backend/environment.d.ts

@@ -14,7 +14,7 @@ declare global {
 			JWT_SIGNUP_SECRET: string;
       MONGO_URL: string;
       NODE_ENV: 'development' | 'staging' | 'testing' | 'production';
-      VERBOSE_ERROR_OUTPUT: boolean;
+      VERBOSE_ERROR_OUTPUT: string;
       LOKI_HOST: string;
       CLIENT_ID_HEROKU: string;
       CLIENT_ID_VERCEL: string;